1. Field of the Invention
The present invention relates to a control system for a synchronous motor adapted to be driven by a frequency converter. Particularly, the invention concerns a control system for suppressing a pulsation in torque or a torque ripple produced in a synchronous motor driven at a low rotation speed thereof.
2. Description of the Prior Art
The synchronous motor adapted to be driven by a thyristor converter is well known in the art as referred to as a commutatorless motor which is a variety of a variable speed motor having no commutator and allows advantageously a very wide speed control range as well as a facilitated maintenance due to the commutatorless structure.
In general, the commutatorless motor is classified into two types, i.e. current type commutatorless motor and voltage type commutatorless motor. The current type motor is adapted to be driven by a square current. The torque produced in this type commutatorless motor is likely to contain pulsating or ripple component. On the other hand, in the case of the voltage type commutatorless motor, no pulsating or ripple component will be theoretically present. However, when unbalance occurs among the armature currents of different phases or when the current waveform undergoes distortions, the ripple component will be present in the torque as produced. Among the causes for involving such waveform distortion, there may be enumerated, for example, amplitude limitation of the armature current performed with a view to enhancing the utility efficiency of the thyristor elements constituting the driver or converter circuit, as discussed in U.S. Pat. No. 3,942,091. It will be readily appreciated that such ripple components in the produced torque will result in fluctuation of the motor speed particularly at a low revolution frequency and exert adverse influence onto a load powered from the motor. Further, occurrence of resonance in the mechanical system including the rotating shaft will sometimes incur application of an excessive torque on the motor output shaft.
With an attempt to eliminate the disadvantages described above, U.S. Pat. No. 3,919,609 teaches a method and circuit for reducing the torque ripple in which a torque produced is arithmetically determined on the basis of a product of the motor voltage and current and utilized as an actually detected torquue value which is then compared with a command or reference torque value. The motor current is controlled in dependence on the difference or deviation obtained from the comparison in the sense to reduce the torque ripple. However, this prior art control can not evade a drawback ascribable to the fact that the torque as produced is arithmetically determined from the product of the armature voltage and current. More specifically, the armature voltage (i.e. induced voltage) will be at a low level in a range of low revolution frequencies in which the torque ripple comes to the front. On the other hand, there are voltage components which play no part in the production of torque, e.g. voltage component induced by the ripple contained in the rectified output voltage of the frequency converter and the resistive voltage drop produced in the motor winding. Although these voltage components are subjected to substantially no variation as a function of the motor rotation speed, they will occupy a relatively large portion of the motor voltage at the low rotation speed. Consequently, the accuracy at which the torque difference or deviation is detected will be necessarily degraded in the low speed operation of the motor to such a degree that satisfactory suppression of the torque ripple can not be effected. Further, because the actual torque value will become pulsated due to the voltage component induced by the ripple component contained in the rectified output from the converter circuit, the motor current will undergo a corresponding pulsation, which results in an increased torque ripple. In this manner, it is impossible to suppress the torque ripple at a low rotation speed of the motor with a desired accuracy in accordance with the teachings disclosed in U.S. Pat. No. 3,919,609.